1. Field of the Invention
The present invention relates to a battery state monitoring circuit that monitors a state of a battery, and a battery device that is equipped with a plurality of the battery state monitoring circuits.
2. Description of the Related Art
For example, JP 2005-117789 A discloses a protective IC that monitors voltages of a plurality of batteries that are connected in series with each other. FIG. 15A shows an example of the protective IC that is disclosed in JP 2005-117780 A. Referring to FIG. 15A, reference numbers 31a, 31b, and 31c denote protective ICs, respectively. The protective IC 31a monitors the voltages of batteries 1a to 1c, the protective IC 31b monitors the voltages of batteries 1d to 1f, and the protective IC 31c monitors the voltages of batteries 1g to 1i, respectively. In a normal state, that is, when the voltages of the batteries 1a to 1i are not abnormal, because all FETs 51, 53, and 55 of the respective protective ICs 31a, 31b, and 31c are on, a current flows through a resistor 81, and a monitor output terminal 42 becomes at high level. On the other hand, for example, when the voltage of any one of the batteries 1a to 1c becomes overvoltage (overcharged state), a signal of high level is output from an overvoltage detector circuit 34a′ that is disposed in the protective IC 31a with the results that an FET 73 is turned on, and an FET 75 is turned on. In this situation, because the FET 51 is turned off, no current flows in the resistor 81, and the monitor output terminal 42 becomes at low level. The same is applied to overdischarge detection.
As described above, when the voltage of any one of the batteries 1a to 1c becomes overvoltage, the monitor output terminal 42 becomes at low level because the FET 73 is turned on, the FET 75 is turned on, and the FET 51 is turned off. However, a parasitic diode having an anode terminal connected to a drain terminal of the FET 51 and a cathode terminal connected to a source terminal of the FET 51 exists between the drain terminal and a gate terminal of the FET 51. Therefore, when a load is connected between external terminals 41 and 44 in the above state, a current path is formed as shown in FIG. 15B, which leads to such a problem that electricity is discharged from the batteries 1d to 1i to generate discharge leak current.
The voltages of the batteries 1d to 1i are decreased due to an influence of the above discharge leak current, but the other batteries 1a to 1c have the high voltage close to the overvoltage. As a result, the voltage balance of the batteries 1a to 1i is disrupted. A state in which the voltage balance is disrupted is advanced so that the batteries 1a to 1c become voltages close to the overvoltage, and the batteries 1d to 1i become voltages close to overdischarge. As a result, because the overvoltage is detected by small charge, charging cannot be conducted. Also, because the overdischarge is detected by slightly using an application program, the batteries cannot be used. Such batteries are exchanged with fresh batteries. However, because the phenomenon of the discharge leak current is repeated so far as the conventional protective IC is used, the conventional protective IC not only causes inconvenience for a user, but also causes a large load such as costs and time required for battery replacement.